The present invention relates generally to electromagnetic interference (EMI) and, more particularly, to filtering EMI from low frequency transmission lines at a device enclosure.
Radio frequency (RF) interference and other emitted radiation is a known problem, particularly in the field of computer systems and other electronic devices. Global standards have been developed and are in use specifying acceptable ranges and amplitudes of permitted emitted interference. Yet it has proven difficult to effectively and inexpensively restrict or otherwise limit undesired interference in compliance with the standards.
Many computer devices and other electronic devices have high-speed clock circuits and switching circuitry, that demand high-levels of instantaneous current, causing high frequency radiated emissions. Electrical cables in proximity to that circuitry receive these emissions, and can transmit or otherwise carry the emissions. If not removed or at least decreased, such emissions can be emitted by the cables outside the device chassis, particularly by cables that penetrate through the chassis. For example, cables that provide input/output and power are generally implicated as radiation sources.
Conventionally, some emission mitigation is afforded for AC power transmission lines (e.g., power cords) through use of a combination of customized line filters and circuit isolation inherent in the design topology of AC power supplies. However, systems that require DC power directly, such as telecommunication systems, do not have the circuit isolation advantage found in AC supplies. As a result, DC power cables and DC powered servers have been notoriously noisy sources of destructive radio interference emissions.